It takes 40 minutes for 9 people to paint 9 walls. How many minutes does it take 15 people to paint 15 walls?
Solution: Imagine that each person is assigned one wall, and all 9 people begin painting at the same time. Since everyone will finish painting their assigned wall after 40 minutes, it takes one person 40 minutes to paint one wall. If we have 15 people and 15 walls, we can again assign one wall to each person. Everyone will take 40 minutes to paint their assigned wall. In other words, it takes 40 minutes for 15 people to paint 15 walls.